Younghae Kwon

Towards a new class of heavy ion doped magnetic semiconductors for room temperature applications

The article presents, using Bi doped ZnO, an example of a heavy ion doped oxide semiconductor, highlighting a novel p-symmetry interaction of the electronic states to stabilize ferromagnetism. The study includes both ab initio theory and experiments, which yield clear evidence for above room temperature ferromagnetism. ZnBixO1−x thin films are grown using the pulsed laser deposition technique. The room temperature ferromagnetism finds its origin in the holes introduced by the Bi doping and the p-p coupling between Bi and the host atoms. A sizeable magnetic moment is measured by means of x-ray magnetic circular dichroism at the O K-edge, probing directly the spin polarization of the O(2p) states. This result is in agreement with the theoretical predictions and inductive magnetometry measurements. Ab initio calculations of the electronic and magnetic structure of ZnBixO1−x at various doping levels allow to trace the origin of the ferromagnetic character of this material. It appears, that the spin-orbit energy of the heavy ion Bi stabilizes the ferromagnetic phase. Thus, ZnBixO1−x doped with a heavy non-ferromagnetic element, such as Bi, is a credible example of a candidate material for a new class of compounds for spintronics applications, based on the spin polarization of the p states.

Specific Heat Study of GaMnAs

Specific heat was used to study the magnetic phase transition in GaMnAs. Two types of samples were investigated. The sample with a Mn concentration of 1.6% shows an insulating behavior whereas the sample with a Mn concentration of 2.6% is metallic. The temperature dependence of the specific heat for both samples reveals a lambda-shaped peak near the Curie temperature, which indicates a second-order phase transition is occuring in these samples. The critical behavior of the specific heat for the GaMnAs samples is consistent with the mean-field behavior with Gaussian fluctuations of the magnetization in the vicinity of TC.

Magnetic and optical property studies on cubic Gd3Fe5−xCoxO12 nanogarnets for spintronics

Investigations on wide band gap nanocrystalline magnetic materials are the subject of recent research interest for establishing functional spin-based nanodevices. In this regard, gadolinium-based rare earth garnets (Gd3Fe5−xCoxO12) were processed in the form of nanostructures by a facile chemical route involving high-temperature annealing treatments. The garnet configuration and the existence of secondary phase characteristics were identified using Raman and X-ray diffraction analysis, respectively. The average size of the nanoparticles was estimated to be around 50–60 nm using Scherrers formula and further confirmed using scanning/transmission electron microscopy imaging techniques. The wide band gap of Gd3Fe5−xCoxO12 systems was studied using the Tauc plot extracted from UV-vis absorbance measurements. A broad luminescence was also observed along the ultraviolet and green regions of the photoluminescence spectrum, which was attributed to the intermediate defect levels existing within the band gap of the material. The electrochemical characteristics of the Gd3Fe5−xCoxO12 nanostructures were further identified using the Nyquist-type impedance plots. Additionally, the saturation magnetization observed in the room-temperature magnetic (M–H) measurements was attributed to the complex magnetic structure of the garnet. Finally, in the present study, the investigation results suggest Gd3Fe5−xCoxO12 as an ideal candidate for applications in magneto-optical devices and spintronics.

Room temperature transparent conducting magnetic oxide (TCMO) properties in heavy ion doped oxide semiconductor

Bismuth doped ZnO (ZnBi0.03O0.97) thin films are grown using pulsed laser deposition. The existence of positively charged Bi, absence of metallic zinc and the Zn-O bond formation in Bi doped ZnO are confirmed using X-ray Photoelectron Spectroscopy (XPS). Temperature dependent resistivity and UV-visible absorption spectra show lowest resistivity with 8.44 × 10-4 Ω cm at 300 K and average transmittance of 93 % in the visible region respectively. The robust ferromagnetic signature is observed at 350 K (7.156 × 10-4 emu/g). This study suggests that Bi doped ZnO films should be a potential candidate for spin based optoelectronic applications.

Magnetic characteristics of Mn-implanted GaN nanorods followed by thermal annealing

We have investigated the magnetic and optical properties of dislocation-free vertical GaN nanorods with diameters of 150nm grown on (111) Si substrates by radio-frequency plasma-assisted molecular-beam epitaxy followed by Mn ion implantation and annealing. The GaN nanorods are fully relaxed and have a very good crystal quality characterized by extremely strong and narrow photoluminescence excitonic lines near 3.47 eV. For GaMnN nanorods, it can be concluded that the ferromagnetic property of GaMnN nanorod with a Curie temperature over 300 K is associated with the formation of Mn4Si7 magnetic phase which results from the effects of magnetic and structural disorder introduced by a random incorporation and inhomogeneous distribution of Mn atoms in the porous layer between the nanorods that form precipitates in the Si substrate before or during the annealing step amongst the GaN nanorods.